1. Field of the Invention
The present invention relates to a method and to apparatus for preparing a substitution liquid.
2. Description of the Related Art
The term "substitution liquid" is used to designate a liquid for injection into the vascular circuit of a patient suffering, for example, from severe burns or having lost large quantities of blood or else a patient subject to blood purification treatment by hemofiltration or by hemodiafiltration, prescribed after temporary or permanent loss of renal function.
The invention is particularly applicable to treatment of this type since the quantity of substitution liquid required during a hemofiltration or a hemodiafiltration session may be very large (with some patients, as much as 30 liters for a four-hour session).
A substitution liquid must be sterile and pyrogen-free, i.e. it must be free from any pathogen and from any substance that could give rise to a febrile reaction in the patient. In addition, it must be isotonic relative to blood, i.e. it must have the same electrolytic concentration as blood. The relative proportions of cations and anions resulting from the dissociation of the electrolytes in the substitution liquid are adjusted as a function of patient requirements, depending on whether the patient's electrolytic equilibrium needs to be maintained or to be re-established.
Finally, a substitution liquid must contain at least those blood electrolytes which play an essential role in the metabolism, i.e., when dissociated in the form of cations and anions: sodium, potassium, calcium, and magnesium as cations, and chlorides and bicarbonates as anions, with the bicarbonates being essential to perform the buffer agent function.
The characteristics of a substitution liquid as recalled above, namely its sterility, its pyrogen-free nature, its isotonia relative to blood, and its specific electrolytic composition, mean that preparing a substitution liquid is difficult and requires special precautions.
In hospitals, it is common practice to use substitution liquids of standard composition prepared industrially by specialized suppliers. These liquids suffer from the drawback of being expensive. In addition, when they contain bicarbonates, they present storage difficulties that are not fully resolved at present, due to a combination of two factors. The first of these factors is that bicarbonates are unstable and decompose spontaneously and continuously into carbon dioxide when they are not confined in gastight manner. The second of these factors is that the bags used for packaging the substitution liquids are generally gas permeable. As a result, if it is desirable to use a substitution liquid having a precise concentration of bicarbonate, then the liquid must be used very soon after it has been packaged, and this constitutes a constraint on the user.
U.S. Pat. No. 4,702,829 describes apparatus for preparing a substitution liquid specifically for use during hemodiafiltration treatment, which apparatus mitigates these drawbacks in part. In that patent, proposals are made to prepare a substitution liquid on the basis of a dialysis liquid manufactured in a dialysis liquid generator, with dialysis liquids generally having substantially the same composition as standard substitution liquids but being neither sterile nor pyrogen-free. The apparatus described in that patent comprises a branch on the dialysis circuit, which branch includes two sterile filters between which a circulation pump is disposed.
In addition to being unsuitable for responding to a very specific requirement, that apparatus suffers from the drawbacks inherent to using expensive filters that must be changed regularly and whose possible clogging must always be detected in time.